Fiber optic light conductors are finding increasing use in various kinds of data communication systems, since an optical signal can be confined to the fiber optic conductor and thereby isolated from external optical interference. In designing and servicing such optical communication systems, it is frequently necessary to know the power of the signal being transmitted on the fiber optic light conductor in order to measure such parameters as system gain or attenuation. Prior art optical measuring instruments usually measure only radiant flux, that is, the power passing through some unit area. An example of a prior art radiant flux measuring instrument is shown in an article entitled "Optical Power Measurements Made Easy" by C. L. Hicks and M. R. Mellon in the Hewlett-Packard Journal, July, 1971, pages 10-16. While such measurements are useful in some optical applications they are not particularly well suited to providing desirable information about the power transmitted in a fiber optic cable since it is usually desirable to know the absolute value of the total power transmitted by the fiber optic cable.